eVeryBOdy lIKeS MOney. It is very easy to spend.With cash and credit cards, you can buy what you wantwhen you want it. So why are quantum computingtheorists trying to rethink money?

There are a few things we all take for granted about
money. We trust credit card companies to keep our
transactions private and to send the right amount of
money to the right place quickly. When we use paper
money, we are used to the fact that we have to carry it
physically with us, and we accept the risk of occasional
counterfeiting.

Today, we use two basic kinds of money. First, there
is the kind we carry around—coins, bank notes, poker
chips, and precious metals. These are objects that are
made by a mint or dug out of the ground. It is easy to
verify that money is valid. You can look for the security
features on paper money, you can feel coins in your
hand, and, if you really know what you are doing,
you can assay precious metals. All of these kinds of
physical money can be counterfeited, though—if you
have the right equipment, you can print paper money,
stamp out your own coins, or make alloys or platings

that look a lot like solid precious metals. Some subway passes and copy
cards are also examples of physical
money—they contain small computer
chips or magnetic strips that are actually worth a certain number of subway
rides or copies. But these tend to be
even easier to counterfeit. 22 In theory,
any physical money can be counterfeited by just using the same production process as the one used to make
the original.

The other kind of money is the
kind that you entrust to someone else.
Think bank accounts and credit lines.
You can carry these around with you in
the form of checks, credit cards, and
debit cards—portable devices that let
you instruct your bank to move money
on your behalf. Unlike physical money,
there is no point in copying your own
credit card (it would not double the
amount of money in your bank). With
a credit card, you can carry as much
value as you want without weighing
down your pockets and you can send
money across the globe nearly instantaneously. But credit cards have disadvantages: every time you pay someone,
you need to tell your bank whom to
send money to. This leaves a paper trail
and does not work if your connection
to your bank is down.

Neither of these kinds of money is
ideal. For example, imagine that you
are going to Las Vegas on a business
trip and you want to play some high-stakes games at night. You might feel
conspicuous carrying a fat wad of cash
around the strip. If you use a credit

key insights

Any digital good can be perfectly copied.
this is a major headache for software
companies (and for the entertainment
industry), and is the reason that digital
cash does not exist.

the quantum mechanical “no-cloning’
theorem means that in principle it is
possible to design quantum systems
that cannot be copied. Several recent
works propose to use such systems
for digital money.